1. Field of the Invention
The present invention relates to a sheet-decorating injection molding apparatus for forming a molding decorated with patterns and letters by bonding a decorative sheet to the surface of a resin molding in a mold when producing the resin molding by injection molding, and a sheet-decorating injection molding method for the sheet-decorating injection molding.
2. Description of the Related Art
Various sheet-decorating injection molding methods which bond a decorative sheet to the surface of a resin molding in a process of forming the resin molding in a mold by injection molding have been proposed. Most of those previously proposed sheet-decorating injection molding methods carry out all or some of the following steps (a) to (j) in that order, in order of different sequence of those steps, or carry out a plurality of steps among those steps (a) to (j) simultaneously or in parallel.
Step a: A sheet feed process for feeding a decorative sheet into a space between a movable mold (usually, a female mold) and a fixed mold (usually, a male die) forming an injection mold
Step b: A sheet fixing process for fixing the decorative sheet to the parting surface of the movable mold by pressing the decorative sheet with a sheet clamper
Step c: A sheet cutting process for cutting off the decorative sheet located on the parting surface of the movable mold from a web if decorative sheets are fed in a web unwound from a roll
Step d: A sheet softening process for softening the decorative sheet by heating the same by a hot plate or the like
Step e: A sheet drawing process for drawing the decorative sheet so that the decorative sheet extend along the inner surface of the female mold defining a cavity by vacuum forming or pneumatic forming (Preforming process)
Step f: A mold clamping process for moving the movable mold toward the fixed mold to close the injection mold
Step g: An injection molding process for forming a molding by injecting a fluid resin (molten resin) through the fixed mold into the cavity defined by the movable mold and the fixed mold, and making the fluidized resin injected into the cavity
Step h: A mold opening process for separating the movable mold from the fixed mold to open the injection mold
Step i: A trimming process for removing unnecessary portions of the decorative sheet from a portion of the same bonded to the resin molding
Step j: A molding removing process for removing a sheet-decorated molding from the injection mold
A plurality of processes among the foregoing processes may be carried out simultaneously. For example, the sheet fixing process of Step b and the mold clamping process of Step f can be achieved simultaneously by fixedly holding the decorative sheet between the movable mold and the fixed mold by the mold clamping process of Step f. The sheet drawing process of Step e and the injection molding process of Step g can be achieved simultaneously if the decorative sheet is drawn by the heat and pressure of the molten resin injected into the cavity in the injection molding process.
Either a laminated decorative sheet or a transfer decorative sheet is used selectively as the decorative sheet according to the type of the sheet-decorated molding. If a laminated decorative sheet is used, the laminated decorative sheet is bonded integrally to the surface of the resin molding in a decorative layer. If a transfer decorative sheet is used, only a transfer layer, i.e., a decorative layer, of the transfer decorative sheet is transferred to the resin molding to form a decorative layer on the resin molding and a base sheet of the transfer decorative sheet is separated from the transfer layer.
When forming a sheet-decorated molding by a conventional sheet-decorating injection molding apparatus, the sheet clamper is held fixedly between the movable mold and the fixed mold during a period between the completion of the mold clamping process and the start of the mold opening process and, therefore, in most cases, the decorative sheet is pressed against and held fixedly on the parting surface of the movable mold by the sheet clamper during that period. Usually, the sheet-decorated molding, i.e., a resin molding solidified in the cavity and integrally provided with the decorative sheet, remains on the fixed mold, particularly when the fixed mold is provided with a cold runner, when the mold opening process is started, and portions of the decorative sheet fixedly held on the movable mold by the sheet clamper, i.e., portions of the decorative sheet not bonded to the resin mold, are separated from the fixed mold as the movable mold is retracted. Consequently, the decorative sheet is pulled so as to be peeled off the resin molding and there is a possibility that portions of the decorative sheet corresponding to the edges of the resin molding tend to be separated from the resin molding or the decorative sheet tends to be broken. Such a trouble is particularly serious with the laminated decorative sheet because the partial separation or the breakage of the laminated decorative sheet is connected directly with making the sheet-decorated molding defective.
FIGS. 8 and 9 illustrates an arrangement for avoiding such a trouble proposed in JP-A No. 6-315950. In this previously proposed arrangement, a movable mold 12 provided with a molding recess 13 and having a parting surface 12a is moved toward and away from a fixed mold 25, a sheet clamper 39 having the shape of a rectangular frame is disposed between the movable mold 12 and the fixed mold 25 to hold a decorative sheet S on the parting surface 12a of the movable mold 12, and a groove 29 for receiving the sheet clamper 39 therein is formed in the fixed mold 25 at a position corresponding to the sheet clamper 39 in a depth sufficient to permit the movement of the sheet clamper 39 between a position indicated by continuous lines in FIG. 9 to hold the decorative sheet S fixedly on the parting surface 12a of the movable mold 12 and a position indicated by alternate long and two short dashes lines in FIG. 9 to be separated from the decorative sheet S in a state where the movable mold 12 and the fixed mold 25 are joined together.
The sheet clamper 39 is pressed through the decorative sheet S against the parting surface 12a of the movable mold 12 at the completion of the mold clamping process, and sheet clamper 39 is retracted in the groove 29 to be separated from the decorative sheet S at the start of the mold opening process. Since the decorative sheet S is released from the sheet clamper 39, the decorative sheet S is able to be separated from the movable mold 12 and remain on the fixed mold 25 when the movable mold 12 is moved away from the fixed mold 25, so that the decorative sheet S may not be peeled off the resin molding Pa or may not be broken.
Although the sheet clamper 39 fixedly holding the decorative sheet S on the movable mold 12 is separated from the decorative sheet S at a timing during a period between the completion of the mold clamping process and the start of the mold opening process, a marginal portion Sb of the decorative sheet S around an effective portion Sa of the decorative sheet S bonded to the resin molding Pa, i.e., a portion of the decorative sheet extending between the frame-shaped sheet clamper 39 and the parting surface 12a of the movable mold 12, is caught by the sheet clamper 39 as shown in FIG. 8 as the movable mold 12 is moved away from the fixed mold 25. Consequently, a part of the effective portion Sa bonded to a portion of the resin molding Pa near the edge E of the sheet-decorated molding P is pulled intensely toward the movable mold 12 by the sheet clamper 39 moving away from the fixed mold 25 as the movable mold 12 is moved away from the fixed mold 25, and the portion of the decorative sheet S bonded to the resin molding Pa is separated from the portion near the edge E or broken.
The foregoing trouble may be avoided if a decorative sheet is held in place on the movable mold by, for example, suction without using the sheet clamper. However, if a decorative sheet were held by suction on the movable mold without using a mechanical sheet clamper, it would be possible that the decorative sheet cannot be satisfactorily drawn by the drawing process due to air leakage, or the decorative sheet is dislocated from a correct position on the movable mold and the decorative pattern of the decorative sheet cannot be correctly formed on the sheet-decorated molding.